1. Field of the Invention
The present invention relates to a method for extracting time information from a received, amplitude-modulated time signal, wherein the time signal is demodulated, the analog signal thus obtained is digitized by a comparator, and the digital signal thus obtained is analyzed to extract the time information.
The present invention also relates to a circuit arrangement for receiving and extracting time information from amplitude-modulated time signals transmitted by a time signal transmitter, in particular for carrying out the inventive method, having a demodulator for demodulating the time signal and generating an analog signal from the time signal, having a comparator for digitizing the analog signal obtained, and having an analysis unit to extract the time information from the obtained digital signal, and also concerns an electronic clock.
2. Description of the Background Art
The radio-controlled transmission of time information is accomplished by time signals, which are emitted by appropriate transmitters, hereinafter referred to simply as time signal transmitters. A time signal is understood to mean a transmitter signal of short duration whose purpose is to transmit the time reference provided by a transmitter. This is a modulating wave, generally having multiple time markers that when demodulated represent only a single pulse that reproduces the transmitted time reference with a particular uncertainty.
The German long-wave transmitting station DCF-77, controlled by atomic frequency standards, continuously transmits amplitude-modulated long-wave time signals on the official atomic time scale CET with a power of 50 KW at the frequency 77.5 kHz. Similar transmitters exist in other countries, transmitting time information on a long-wave frequency in the range between 40 kHz to 120 kHz. All of the aforementioned countries use what is called a telegram, which is precisely one minute long, for transmitting the time information.
FIG. 1 shows the coding scheme (telegram)—labeled with reference symbol A—of the coded time information in the case of the German time signal transmitter DCF-77. The coding scheme in the present case has 59 bits, where 1 bit corresponds to one second of the time frame in each case. In this way, what is known as a time signal telegram can be transmitted over the course of one minute; it contains, in particular, information on time and date in binary encoded form. The first 15 bits B contain general coding, which contains operating information, for example. The next five bits C contain general information. Thus, R designates the antenna bit, A1 designates a flag bit for the transition from Central European Time (CET) to Central European Summer Time (CEST) and back, Z1 and Z2 designate zone time bits, A2 designates a flag bit for a leap second, and S designates a start bit for the coded time information. The time and date information is transmitted in BCD code from the 21st bit to the 60th bit, with the data applying to the next minute in each case. In this context, the bits in area D contain information on the minute, those in area E contain information on the hour, those in area F contain information on the calendar day, those in area G contain information on the week day, those in area H contain information on the month, and those in area I contain information on the calendar year. This information is present bitwise in coded form. Check bits P1, P2, P3 are provided at the end of each of the areas D, E and I. The sixtieth bit of the telegram is not occupied, and serves the purpose of indicating the start of the next frame. M designates the minute marker, and thus the start of the time signal telegram.
The structure and the bit assignment of the coding scheme shown in FIG. 1 for transmitting time signals is generally known, and is described in the article by Peter Hetzel, “Zeitinformation und Normalfrequenz,” (time information and standard frequency) in Telekom Praxis, Volume 1, 1993.
Transmission of the time signal information as shown in FIG. 2 is accomplished with amplitude modulation by individual seconds markers. The modulation includes a decrease X1, X2 (or an increase) in the carrier signal X at the start of each second, with the carrier amplitude being reduced to approximately 25% of the amplitude at the beginning of each second—except for the fifty-ninth second of each minute—for a period of 0.1 seconds X1 or 0.2 seconds X2 in the case of a time signal transmitted by the DCF-77 transmitter. These decreases X1, X2 of different duration each define seconds markers or data bits in decoded form. The different durations of the seconds markers are used for binary coding of time and date, wherein seconds markers with a duration of 0.1 seconds X1 represent a binary “0” and those with a duration of 0.2 seconds X2 represent a binary “1”. The following minute marker is indicated by the absence of the sixtieth seconds marker. In combination with the applicable second, analysis of the time information transmitted by the time signal transmitter is then possible. FIG. 2 uses an example to show a section of such an amplitude-modulated time signal X. However, the analysis of the precise time and precise date is only possible when the 59 second bits of a minute are unambiguously identified and thus a “0” or a “1” can be unambiguously assigned to each of these seconds markers.
In other countries, such as Great Britain, Japan, or the USA, the modulation also is accomplished by reductions or increases in the amplitude of the carrier signal X, but the seconds markers, and thus the time durations of the reductions or increases X1, X2, vary to greater or lesser degrees, and have a length from 100 ms to 800 ms, depending on the transmission protocol. In the American (WWVB) and Japanese (JJY40 and JJY60) protocols, the time proportions of the signal reductions are 50% or 80% of a full second. The level of the reduction is also different in all the protocols, ranging from complete reduction to a level of zero in Great Britain (MSF) to a reduction of only 32% of the nominal amplitude in the American transmitter (WWVB).
For general background on radio clocks and circuit arrangements for receiving time signals, reference is made to DE 198 08 431 A1, DE 43 19 946 A1, DE 43 04 321 C2, DE 42 37 112 A1, and DE 42 33 126 A1. With regard to the extraction and processing of time information from time signals, reference is made to DE 195 14 031 C2, DE 37 33 965 C2, and EP 042 913 B1.
Conventional circuit arrangements of the aforementioned type typically have an (output) comparator, which, for the purpose of digitization, compares the signal level of the rectified analog signal obtained through demodulation to a reference value, the comparator threshold, and as a result supplies either a low level signal (logic “0”) or a high level signal (logic “1”), depending on whether the analog signal level is below or above the comparator threshold. A logic that is inverted with respect to the foregoing is also possible. Since the rectified signal only slowly follows the input amplitude of the actual input signal, however, and the rising and falling edges often do not have the same steepness, the position of the comparator threshold has a strong influence on the time duration of the low phase (“0”) or high phase (“1”) of the digital signal produced. Especially in the case of low input levels, which are always overlaid with noise, the edges of the analog signal—especially the falling edges—are relatively flat and the absolute voltage changes are relatively small, so that the comparator threshold has a particularly strong influence on the time durations of the low and high phases of the digital signal.
The position of the threshold of the output comparator accordingly plays a critical role in the conversion of the rectified and demodulated analog signal into a digital signal, on which basis—as described above—the reductions of the amplitude-modulated time signal or their time durations are determined by the analysis unit for the purpose of extracting the time information contained therein. Nowadays the comparator threshold for the analog decoded time signal (TCO) is disadvantageously either set at a fixed level or is governed in an analog manner by the input amplitude. In this connection, it appears useful for low input levels to increasingly move the comparator threshold toward a maximum level of the analog signal, and to correspondingly lower it at high input levels. This approach has already been implemented in some types of integrated receiver circuits. However, such a tracking of the comparator threshold must be designed in a relatively modest manner, since no feedback is provided. In addition, this regulation as such must also be viewed as disadvantageous, since only the signal amplitude, and not the signal itself (the signal quality), is analyzed, which can lead to an erroneous reception of the time information. “Erroneous” means that during the duration of a received minute record, incorrect binary decisions are made which lead to an incorrect evaluation of at least one data bit of the minute record. The time derived from the received time signal would then no longer be correct.